* model_summary.md - Add link to Harvard's Annotated Transformer.
* model_summary.md - slight wording change + capitalize name of the paper
* model_summary.md - moves the Annotated Transformer link in a praenthesis next to the link to the original paper (great idea, stevhliu!)
* model_summary.md - moves the Annotated Transformer link in a praenthesis next to the link to the original paper (commit pt. 2, accidentally removed "has" in pt. 1)